This invention relates to a diode-pumped solid-state ring laser gyro and, more particularly, to a diode-pumped monolithic solid-state ring laser which uses the total reflection on the boundary surfaces for the deviation of the resonator mode.
A currently wide-spread method for measuring the rotating movement of a moved apparatus--for example, of a vehicle, a ship, an airplane or a satellite--is the use of ring laser gyros (RLK). The gyro is used for the control and stabilization of the movement, the "northing", as well as for the calibration of acceleration sensors in inertial navigation systems. For the wide-area navigation in the airplane, a long-term stability of better than 0.01.degree./h is required; for the ship navigation, even 0.001.degree./h; however, for measurements in combination with other navigation methods, such as the Global Positioning System (GPS) as well as for short-term measurements of rotational movements in the vehicle, the missile and the combat airplane, the requirement is at 10.degree./h to 100.degree./h. The measuring range is normally between 0-10.degree./s. Since the ring laser gyro always detects only one axis of rotation because of its planar construction, three gyros are required which are situated perpendicularly with respect to one another for measurements in all directions in space.
The physical basis of the ring laser gyro is the Sagnac effect which describes the influence of a rotational movement on the propagation of light waves. When a light wave is deviated by 360.degree. by a mirror reflection or in an optical wave guide and is caused to superimpose on itself, ring waves are created. Since both rotating directions are equivalent, a left-rotating as well as a right-rotating ring wave can form simultaneously. When the wave-guiding structure is rotated, the frequency of the moving wave will increase and the frequency of the wave moving in the opposite direction will decrease. Particularly in the ring laser gyro (RLK), the opposed waves in the ring are continuously optically amplified. Simultaneously, a portion of the two waves is coupled out of the ring by means of a dividing mirror and is superimposed on a photo detector for measuring the difference frequency. The difference frequency is proportional to the rate of rotation .OMEGA. and proportional to the area A enclosed by the waves, but inversely proportional to the light path L and the wave length .lambda. in the amplifying medium: EQU .DELTA..nu.=4A.OMEGA./L .lambda. (1)
When a laser beam of the wave length .lambda.=0.63 .mu.m is deviated along the sides of a square with a side length of 4 cm, the frequency shift (with the rate of rotation .OMEGA. 15.degree./h) caused by the rotation of the earth is .DELTA..nu.=4.4 Hz. A rotating speed of 500.degree./s, which may occur during a rolling motion of a combat plane, will supply .DELTA..nu.=400 kHz as the gyro signal.
The commercially available laser gyros use HeNe gas lasers. The resonator is designed either as an isosceles triangle with three deviating mirrors or as a square with four deviating mirrors. Two gas discharge tubes along the beam path provide the laser amplification at the wave length 0.633 .mu.m or 1.152 .mu.m. So that the structure remains as mechanically and thermally stable as possible, the laser is usually integrated in a block made of a material with an extremely low coefficient of expansion. The deviating mirrors are mounted on the corners in a vacuum-tight manner. The bores are evacuated and are filled with the HeNe mixture to a pressure of a few torr. The gas discharge is ignited between two electrodes.
The deviating mirrors form the optical resonator. As in the case of a longitudinal resonator, one or two of the mirrors are spherically curved; the others are planar. A deviation by 60.degree. (isosceles triangle) or by 90.degree. (square) in the plane on each mirror provides for the formation of a closed ring wave. However, for this purpose a very precise alignment of the mirrors is required, as in the case of the linear resonator.
One of the deviating mirrors is optically semireflecting. A portion of the two ring waves is coupled out and both are guided together in a special deviating prism on a photodetector where, with a slight inclination of the wave fronts, they form an interference band pattern. The Sagnac effect, which occurs during a rotation of the gyro, is detected as a movement of the interference bands over its sensitive surface. By means of the moving direction of the interference bands, which is detected by means of a double photodiode, the rotating direction of the gyro can be clearly determined.
The angle of slope of the two beams .phi. and the size of the photodetector surface d.sub.d is adapted to one another such that the diameter of the detector corresponds approximately to the distance between two interference minima d.sub.i : EQU d.sub.d .about.d.sub.i =.lambda./2 sin .phi. (2)
It is the object of the photodetector to count the number of intensity bands N which is proportional to the angle of rotation .theta., with ##EQU1##
So that the interference pattern remains spatially stable, the laser must operate in a transversal fundamental mode and simultaneously on a single longitudinal mode of the resonator. The fundamental mode and its spatial stabilization is forced by the limiting of the beam path by means of screens. The longitudinal monomode operation occurs automatically in the case of a forming closed ring wave.
The absolute position of the laser frequency must be maintained to be very stable so that no measuring errors occur because of frequency fluctuations which, although they have the same effect for both ring waves, on the whole contribute to the noise background. This is carried out by adjusting the position of the mirrors by means of piezo-actuators to a fixed frequency value. Faulty measurements which may occur are bias errors because of a non-uniform amplification of the two opposed ring waves, a difference in the course of the two optical beam paths and an asymmetry in the ion transport in the gas discharge (Langmuir flow).
The best-known disturbance in a ring laser gyro is the so-called "frequency lock-in" as the result of an optical cross-coupling between the opposed ring waves. During the reflection of the laser waves on the multi-layer dielectric deviating mirrors, a small fraction of scattered light occurs on the surface which is partly scattered back into the opposite direction in the beam path. This scattered light is further amplified by the laser process and now competes with the second measuring wave. With the very high frequency of the laser waves, for example, .nu.=4.7.times.10.sup.14 Hz at .lambda.=0.63 .mu.m, and a measuring difference frequency in the range of Hz to several tens of kHz, the phenomenon of the "frequency lock-in" of two oscillators of a comparable frequency will occur, which is generally known in the field of electronics. The ring waves shift toward a common frequency (injection locking) and the difference frequency--the actual measuring frequency--will disappear.
Since the "lock-in" does not occur before the lower frequency range, this problem can be reduced or eliminated by the artificial displacement of the difference wave lengths (that is, targeted biasing) into a higher frequency range. In the commercially available apparatuses, three different methods are used for this purpose. The first consists of causing the whole ring laser gyro structure to rotate at a fixed rotating speed (Raytheon Co., USA). In the second, the structure is caused to carry out a periodic vibration (dithering--Honeywell Co., USA). The third method (Northrop Co., USA) uses magneto-optical deviating mirrors. By means of the transversal Kerr effect in thin magnetic mirror layers of the deviating mirrors, as a result of the switching-over of a magnetic field, a periodic phase shift is exercised on the deviated ring waves.
Basically, the ring laser gyro on the basis of gas lasers has the following disadvantages. (1) Miniaturizing is possible only up to a limited degree because a certain minimum amplifying length is required. (2) The laser structure is mechanically and thermally sensitive. (3) The use of high voltage for the operation of the gas discharge is disadvantageous (tube technology). (4) The overall efficiency of the laser is very low with &lt;0.01% (thermal stress). (5) The manufacturing of mirrors which are extremely low in scattered light for suppressing the "lock-in effect" is high in cost. (6) Screens for limiting and stabilizing the beam path generate scattered light which promotes the "lock-in effect". (7) The rotating or wobbling of the ring laser gyro in order to bypass the "lock-in effect" during the measuring results in high technical expenditures, is expensive and generates mutual disturbances of the measurements in three axes. (8) The elimination of asymmetries in the gas transport in the discharge tube requires very high technical expenditures. (9) The magnetic field for generating the Kerr effect in magneto-optical mirrors simultaneously induces a disturbing Zeeman line splitting of the laser line in the gas discharge.
Despite these high technical expenditures, the ring laser gyro with the HeNe-laser is in very wide-spread use as a component of inertial platforms in airplanes and ships. In the case of much lower requirements on the angular resolution (&gt;10.degree./h), as in missiles, for the directional control of robots and automatic machines, recently the so-called passive fiber gyro has increasingly been used where the Sagnac effect results in the phase shifting of oppositely rotating light waves in glass fibers. This technology has the important advantage that opto-semiconductors of a low frequency quality, long service life and low cost can be used as the light source. A disadvantage is the thermal and mechanical sensitivity of the relatively unwieldy large fiber coil and the low achievable angular resolution.
Furthermore, monolithic ring lasers have been known for some time in literature (see, for example, T. J. Kane and R. L. Byer, "Monolithic, Unidirectional Single-Mode Nd:YAG Ring Laser", Optics Letters, Vol. 10, No. 2, Page 65 (1985) and are already offered on the market by Lightwave Co. in the U.S. as particularly frequency-stable laser sources (MISER). Such lasers are the object of intensive research because they can supply a single-mode very narrow-band radiation for various applications into the power range close to 1 W (see, for example, I. Freitag, P. Rottengatter, A. Tunnermann and H. Schmidt, "Frequency Adaptable Diode-Pumped Miniature Ring Lasers", Laser und Optoelektronik 25 (5) 1993). It is decisive for the applications of these lasers that the second ring wave is suppressed by the installation of an optical diode in the beam path, for example, by means of the magneto-optical effect.
Despite progress in satellite navigation as the result of GPS and the introduction on the market of new processes for measuring the rate of rotation, as, for example, by measuring the effect of the Coriolis force on tuning fork vibrations, there is a wide demand today for the ring laser gyro as an alternative measuring process or as a measuring process supplementing the GPS in which, for example, interruption times in contact with the satellite must be bridged. Additional applications are for the measuring of short-term rotating movements in the vehicle and in machines. Although the technique of the gas laser ring laser gyro is fully developed, the technical expenditures for a wide application in the future are too high. Their manufacturing is too cost-intensive and their useful life is limited to a few thousand operating hours. The low-cost miniaturized semiconductor lasers may be conceivable as laser amplifiers in open ring laser mirror structures but, because of the low frequency stability and poor radiation quality, they are unsuitable for the implementation of a high-quality ring laser gyro and are too expensive for lower-quality demands. Because of its poor measuring resolution, the fiber gyro is a replacement only for certain applications and can basically not have a small construction
There is therefore needed a diode-pumped solid-state ring laser gyro which is not only simplified and miniaturized in its mechanical construction but also has no movable parts; is insensitive to external disturbances; with respect to the measuring resolution and measuring precision corresponds to a high-quality HeNe ring laser gyro, and permits a simultaneous rate of rotation measurement about all space axes in a monolithic gyro body.
These needs are achieved by a diode-pumped monolithic solid-state ring laser which uses the total reflection on the boundary surfaces for the deviation of the resonator mode. The solid-state ring laser is formed as a gyro in that two ring waves rotating in opposite directions in a common solid-state laser material are simultaneously amplified. From the difference frequency of the two ring waves, a rate of rotation of the now solid-state ring laser gyro is derived. The solid-state medium itself consists of doped crystal or glass material of a preferably square or rectangular shape.